1. Technical Field
The present invention relates to a laser device, more specifically to a laser device used for a driver of an EUV light source.
2. Related Art
To achieve higher density LSI, the photolithography technology, in which a circuit pattern is optically transferred on a semiconductor wafer, plays an important role. In the photolithography process, a reduced-projection-exposure device, called a stepper, is mainly employed. In other words, with the reduced projection light system, a transmitted light having an original pattern (reticle) emitted by a light source is projected to a light-sensitive material on a semiconductor substrate, and the circuit pattern is formed. The resolution of this projected image depends on the wavelengths of the light source used. Thus, with the increasing demand for finer pattern line width, the wavelength of the light source is shortened to an ultraviolet range.
Recently, as light sources, KrF excimer laser (wavelength of 248 nm) and ArF excimer laser (wavelength of 193 nm) that generate light at a range of deep ultraviolet (DUV light) have been employed. Furthermore, F2 laser (wavelength of 157 nm) that generates light at a range of vacuum ultraviolet (VUV light) is developed as the light source.
These days, for the purpose of finer machining, EUV light sources (wavelength of 13.5 nm) that output the light at a range of extreme ultraviolet (hereinafter, “EUV light”) have been tried as the light source for the photolithography.
In general, an LPP (laser produced plasma) type and a DPP (discharge produced plasma) type are used to generate the EUV light. In the LPP-type EUV light source, the short-pulse laser light generated and emitted by a driver laser device is projected onto a target formed by tin Sn; the target is excited to be a plasma state to generate the EUV light; and the EUV light is collected by a collection lens to output the EUV light to the outside.
As the driver laser device, a CO2 laser and so on are employed.
Currently, the EUV light having output of 140 W is required for the light source of the exposure device. However, in the LPP-type EUV light source system that employs CO2 laser as the driver laser device and uses a target formed of tin Sn, only about 1-2% of output can be obtained as the EUV light output from the laser light emitted by the driver laser device. Thus, to obtain the output of 140 W of the EUV light required, it becomes necessary to use a driver laser device capable of outputting laser of 10 kW.
Incidentally, the industrial CO2 laser having the output of 20 kW is commercially available. However, this industrial CO2 laser is not the pulse oscillating laser device necessary for the driver laser device for the EUV light source, but is a CW oscillating laser device. To utilize as the driver laser device for the EUV light source, the short pulse oscillation of about 10-100 ns is necessary for the CO2 laser. However, there is no commercially available CO2 laser that can output the 10 kW laser with such short pulse oscillation. Thus, there has been conventionally provided a driver laser device having a configuration in which the short pulse oscillation CO2 laser having small output of about 50-100 W is used as the oscillator, and plural amplification stage lasers (amplifiers) are provided at the rear stage of the oscillator.
To obtain the laser output of 10 kW with the driver laser device, about four stages of the amplification stage lasers (amplifier) are required when the oscillator having the output of 50-100 W is employed. Here, the oscillator has a rectangular-solid-shaped housing (hereinafter, “oscillator unit”). Additionally, the amplification stage laser (amplifier) has a rectangular-solid-shaped housing (hereinafter, “amplifier unit”). Thus, those oscillator unit and four amplifier units occupy a large floor space in the semiconductor manufacturing lines. Furthermore, in addition to the oscillator and the amplifier units, the driver laser device is necessary to be equipped with a group of optical elements comprising each optical element for shaping and transferring the laser light in the optical path among the oscillator and the amplifier units. Moreover, a power supply is necessary separately for the oscillator and the amplifier units.
FIG. 1 shows an example of a configuration in which an oscillator unit 11, four amplifier units 21, 22, 23, 24, a group of optical elements 31 (31A, 31B, 31C, 31D), and power supplies 41, 42, 43, 44, 45 respectively for the oscillator unit 11, the amplifier units 21, 22, 23, 24 are arranged on a floor space in the semiconductor manufacturing line.
As shown in FIG. 1, the oscillator unit 11 and the four amplifier units 21, 22, 23, 24 are arranged such that a surface 11A having a wide area of the housing of the oscillator unit 11, and surfaces 21A, 22A, 23A, 24A each having a wide area of the housing of each of the four amplifier units 21, 22, 23, 24 are in parallel with the floor surface. Additionally, the group of optical elements 31 (31A, 31B, 31C, 31D) and the power supplies 41, 42, 43, 44, 45 are arranged accordingly. Thus, the footprint (occupied space) FP further exceeds 100 m2, and becomes extremely large. It would be acceptable if the purpose is to implement experiments or researches. However, it is practically difficult to reserve a vast area in the floor space in the semiconductor manufacturing line only for the EUV light source, or only for the driver laser device.
The present invention has been made in view of the circumstances above and the first problem to be solved by the present invention is to reduce a footprint in arranging a laser device on a floor space in the semiconductor manufacturing line, etc, even if the laser device comprises a number of amplifier units.
As described above, since the oscillator unit 11 and the four amplifier units 21, 22, 23, 24 are arranged such that the surface 11A having a wide area of the housing of the oscillator unit 11, and surfaces 21A, 22A, 23A, 24A each having a wide area of the housing of each of the four amplifier units 21, 22, 23, 24 are in parallel with floor surface, a total optical path LR passing through the units 11, 21, 22, 23, 24 reaches as long as 40-50 m. Therefore, the whole of the group of optical elements 31 cannot be mounted on the same plate, and hence each of the optical elements 31A, 31B, 31C, 31D has to be arranged in a decentralized manner on vibration isolators 51, 52, 53, 54 separately arranged in given places. This causes a problem that operator bears a large burden because the operator has to walk around each of the separated places to perform adjustment of the alignment of the optical elements, or other maintenance operations, which deteriorates the operating efficiency. Furthermore, due to the increased optical path and decentralized arrangement of the group of optical elements, the alignment of the optical system is largely affected by the stability of the floor surface and vibration of the floor or slight shock during the maintenance. For example, in a case where the optical path reaches 40-50 m, the slight displacement of only 100 μm in the transmission optical system results in the displacement of several mm at around the portion where the laser light is outputted, which hinders the efficient amplification and deteriorates the laser output. This poses a problem that it becomes difficult to guarantee the long-term stability of the laser output in a circumstance where this guarantee is required, such as the semiconductor manufacturing line.
The present invention has been made in view of the above circumstances and the second problem to be solved by the invention is to decrease the burden of the operator by shortening the optical path and centralizedly arranging the group of optical elements into one place to enhance the work efficiency during the maintenance of the group of optical elements, and to stabilize the laser output in a long term by making the group of optical elements less likely to be affected by the vibration and so on.